1. Field of the Invention
The present invention generally concerns flowmeters, and particularly differential pressure flowmeters. The present invention more particularly concerns (i) improvements to variable-area flowmeters, or rotameters, and (ii) flowmeters that are insensitive to changes in fluid viscosity.
2. Background of the Invention
Measuring the flow of liquids is a critical need in many commercial operations.
For instance, beverage dispensing systems mix two or more fluids together in prescribed relative proportions. Such systems may also supply fluids at a prescribed flow rate. When each fluid flows at its prescribed rate for a set period of time a predetermined volume of beverage is dispensed. In order to precisely control either or both proportion and/or dispensed volume, the actual rates of fluid flow must be controlled.
One particular type are of beverage dispensing system is a post-mix soft drink dispenser. The post-mix soft drink dispenser mixes together and dispenses carbonated water and flavored soft drink syrup in a prescribed mix ratio. The dispenser typically injects the water and syrup simultaneously into a mixing chamber. The two fluids are mixed together to form a mixed soft drink that is then dispensed from the chamber through a nozzle into a drinking cup. The two fluids are normally supplied for coextensive time durations, and the mix ratio between the two fluids is typically controlled by using manually-adjustable flow control, or "metering", pins.
A goal of soft drink beverage dispensing is to control the accuracy of the "brix", or the percentage syrup in the mixture, to be within 1% of the desired value. [The term "brix" is an industry term derived from "Brix scale", meaning a hydrometer scale for sugar solutions.]
One prior approach to realizing this goal has been to control the pressure, and thereby the flow rate, of the two liquid components. Although a typical post-mix dispensing system so controlled operates satisfactorily in some situations, it has proven difficult to compensate for variations in the pressure of the carbonated water. Some prior systems have overcome the difficulty of precisely controlling water pressure by including relatively complex and expensive structures for regulating the water pressure. Other systems have sought to maintain a fixed mix ratio by controllably adjusting a syrup valve in accordance with the water's pressure. It is believed, however, that even these systems are unduly sensitive to pressure variations. Also, many of these systems are unduly complex and to require substantial manual adjustments when changing from one type of syrup to another.
Accordingly, another prior approach to realizing a post-mix dispensing system has involved the real-time measurement of the flows of carbonated water and of soft drink syrup. A system for so controlling the dispensing of a soft drink beverage is described in U.S. Pat. No. 4,487,333 for a FLUID DISPENSING SYSTEM. The fluid flowmeter used in the FLUID DISPENSING SYSTEM of U.S. Pat. No. 4,487,333, described in detail in U.S. Pat. No. 4,440,030, is a paddle wheel flowmeter operating over an extended range of flow regimes spanning both turbulent and laminar flow regimes.
However, the flowmeter of U.S. Pat. No. 4,440,030, and many other flowmeters, have output signals that vary with the viscosity of the fluid for which flow is measured. In the case of soft drink syrups, viscosity variations between syrups, and in an individual syrup with changes in temperature, are very great. Syrups can commonly vary in viscosity from .times.2 to .times.20 over the range between 32.degree. F. (0.degree. C.) and 194.degree. F. (90.degree. C.). These large variations cause commensurate variations in the signal outputs of previous viscosity-sensitive flowmeters.
Previous attempts to solve the problem of viscosity sensitivity have used 1) a means for determining the viscosity of the fluid passing through each flowmeter, and 2) a means for adjusting the meter's output signal accordingly. The adjusted signal more accurately indicates the fluid's actual flow rate, and this adjusted signal is more suitably conditioned for use by a fluid flow adjusting means to achieve a prescribed mix ratio. In the FLUID DISPENSING SYSTEM of U.S. Pat. No. 4,487,333, the relationship between temperature and viscosity for a particular fluid (soft drink syrup) is contained in a removable personality module, and used by a microprocessor to control the dispensing operation. This manner of providing information to the FLUID DISPENSING SYSTEM facilitates a reliable conversion of the system for use with fluids having different temperature/viscosity characteristics. Use of the personality module also permits the soft drink dispensing apparatus to be used conveniently with a number of different fluids (e.g., soft drink syrups) having different mixing characteristics, without requiring manual adjustments to be made.
The personality module is a complex, and expensive, scheme for compensating for variations in measured flow with changes in fluid viscosity. The personality module could be obviated if the flowmeter were to be substantially insensitive to variations in fluid viscosity. While such flowmeters exist, they typically do not exhibit sufficient accuracy for use in beverage dispensing. For example flowmeters of the wedge, pitot tube, and elbow meter types exhibit low sensitivity to fluid viscosity variations, but exhibit accuracies on the order of .+-.3-5% of full scale and .+-.5-10% of full scale, respectively.
Flowmeters having (i) essentially no sensitivity to fluid viscosity variations and also (ii) high accuracy do exist. Accurate viscosity-insensitive flowmeters include, for example, the electromagnetic, ultrasonic (doppler), ultrasonic (time-of-travel), mass (coriolis), and mass (thermal) types. Unfortunately, all these flowmeter types are very expensive. Finally, Weir (V-notch) and Flume (Marshall) type flowmeters exhibit very low sensitivity to fluid viscosity variations, while of moderate cost, exhibit only medium accuracy.
Variation in viscosity is a physical characteristic of many fluids, including soft drink syrups, that is difficult, if not impossible, to eliminate. Accordingly, post-mix beverage dispensing would benefit from a low cost, high accuracy, flowmeter that was substantially insensitive to variations in fluid viscosity. The flowmeter in accordance with the present invention will be seen to satisfy this requirement.
Differential pressure flowmeters are particularly pertinent to the present invention. The measurement of differential pressure to infer a liquid's rate of flow is well known.
The basic operating principle of differential pressure flowmeters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root.
Differential pressure flowmeters, like most flowmeters, have a primary and secondary element. The primary element causes a change in the kinetic energy of the flowing fluid, which creates the differential pressure in the pipe. The unit must be properly matched to the pipe size, flow conditions, and the liquid's properties. The measurement accuracy of the element must be good over a reasonable range. The secondary element measures the differential pressure and provides the signal or read-out that is converted to the actual flow value.
A particular variable-area flowmeter, also called a rotameter, is shown in FIG. 1. It is perhaps the least expensive and most reliable of all flowmeters. The variable-area flowmeter 1 has a float 2 that moves up and down in a tapered tube 3. The distance moved is proportional to the flow rate of liquid 4 and the annular area between the float 2 and the wall of tapered tube 3. The height of the float 2 in tapered tube 3 may be read against scale 5. Because flow rate can be read directly on a scale mounted next to the tube, no secondary flow-reading devices are necessary.
Variable-area flowmeters, or rotameters, exhibit significant sensitivity to variations in fluid viscosity. In a variable-area flowmeter, the force in one direction on the float, typically due to gravity, is at all times of use equal to the force in the other direction resultant from the differential pressure across the float (between its top and its bottom) due to fluid flow. Meanwhile, the float also serves to variably occlude the liquid flow channel within which it is resident. When the fluid flow rate is greater then the float will move (rise) in the channel until the area of flow is increased and the differential pressure force of flowing fluid against the float is lowered to again equal the (constant) gravitational force.
Of course, if the liquid is more viscous, meaning more frictional, then an equal flow of a more viscous liquid will exert an increased frictional force on the float, forcing it to move in its channel equivalently as if an increased flow were being experienced. Conversely, a reduction in liquid viscosity simulates a reduced flow rate.
Presently available variable-area flowmeters exhibit significant viscosity sensitivity because they do not eliminate, or compensate for, the above-described changing frictional forces due to variations in liquid viscosity.